Helper to declare a result from a Maybe (the item might be
missing, in which case we return no data). Note that there's some
ambiguity here: in some cases, we mean RSNoData, but in other
RSUnavail; this is easy to solve in simple cases, but not in
nested dicts. If you want to return RSUnavail in case of Nothing
use the function rsMaybeUnavail.

Helper to declare a result from a Maybe. This version returns
a RSUnavail in case of Nothing. It should be used for optional
fields that are not set. For cases where Nothing means that there
was an error, consider using rsMaybe instead.

Generic parameter functions

Returns a field from a (possibly missing) DictObject. This is
used by parameter dictionaries, usually. Note that we have two
levels of maybe: the top level dict might be missing, or one key in
the dictionary might be.